void verticalTraversal(node* head, map<int,int> &levelMap, int level) {
    if(head == NULL)
        return;
    if(levelMap.count(level) == 0)
        levelMap[level] = head->data;
    if(level == 0 || level < 0)
        verticalTraversal(head->left, levelMap, level - 1);
    if(level == 0 || level > 0)
        verticalTraversal(head->right, levelMap, level + 1);
}
void topView(node * root) { 
    if (root == NULL)
        return;
 
    map<int, int> levelMap;
    verticalTraversal(root, levelMap, 0);
    
    for (map<int,int>::iterator it=levelMap.begin(); it!=levelMap.end(); ++it) {
        cout << it->second << ' ';
    }
}